Extruder press units can comprise an extruder body having at least one output gap or mouth which is defined by an output gap-defining adjustable extruder lip and an opposing gap-bounding extruder lip, the output gap being adjustable with the aid of a mechanical course adjusting mechanism to a given gap width and a servomechanism is connected to the gap-defining adjustable extruder lip to regulate the thickness of the extruded plastic material. The extruder unit may be constructed as a so-called broad-slit nozzle or die for extrusion of plates and foils, or of suitable shape for extrusion of sections (structural shapes or profiles), hollow sections, pipes and tubes or for foil blowing. Commonly each extrusion press is equipped with a plurality of adjustable elements arranged in a row. The singular will continue, however, to be used herein when referring to the adjustable elements.
In the known extruder unit taught in U.S. Pat. No. 3,040,221, for example which is constructed as a broad-slit nozzle, the adjustable element is constructed as a metal bolt, whose head has a thread and is screwed in a corresponding hole of the extruder body, whose shaft however is heatable by resistive electric heating of an appropriate heating element.
Its thermal expansion determines the gap size adjustment of the adjusting element associated with the control and adjusting device, which permits fine control and adjustment.
The coarse adjustment takes place in a complicated way by screwing the head provided with the thread into the associated hole and cannot be effected free of consideration of the allowed tolerances.
The coarse adjustment and the fine adjustment are therefore coupled in that the allowed variation of the coarse adjustment influences the precision of the fine adjustment because the heating element is part of the control and adjusting device. In practice the bolt shaft is adjusted to a temperature within a particular temperature range and is held at this temperature, until the control and adjustment requires another gap adjustment and therefore requires another temperature of the bolt shaft.
This system has several drawbacks. On the one hand for physical reasons the temperature rise of the bolt shaft occurs only slowly, so that for control and adjustment a lengthy adjusting element equilibration time with its known disadvantages must be tolerated or taken into account.
On the other hand unavoidable temperature inhomogeneities of the bolt shaft impair the precision of this control and adjustment, chiefly because the extruder is heated as a whole and thus control and adjustment fluctuations result from thermal coupling.